A. Field of the Invention
This invention relates to an endoscope with an improved optical system and a process of manufacturing the optical system.
B. Description of the Prior Art
An objective optical system is fitted in the head part of an insertion section of a typical endoscope. For example, in the conventional endoscope disclosed in Japanese Utility Model Disclosure No. 53-121844, an objective optical system fitted in the head of an insertion section. This optical system comprises a first objective lens placed at the forefront and a group of objective lenses placed at the rear of the first objective lens. The first objective lens is mounted air-tightly on a mount, with its front face exposed. At the rear of the first objective lens, a group of objective lenses other than the first objective lens are mounted on a mount.
A conventional objective optical system has a drawback. When the objective lenses are subjected to rapid changes in temperature, dew condenses on the inside face of a first objective lens, making observation difficult. For example, when the front face of the first objective lens is washed, the lens is cooled rapidly from a temperature near the human body temperature. Water vapor may have entered the internal space of the objective optical system little by little through a covering of the inserting section of the endoscope. If this is the case, the high-temperature humid air around the objective lenses is cooled, and dew condenses on the inside face of the first objective lens, thus clouding the lens.
A technique has been proposed to prevent the clouding of a lens due to condensation of water vapor. In this technique, a first objective lens and a second objective lens are mounted airtightly to a lens frame, thereby airtightly sealing the space between the two objective lenses. Specifically, a first objective lens is set and glued to one end of a lens frame. In this way, the first objective lens is secured airtightly. Then, another lens is inserted into the lens frame through an opening at the other end of the lens frame. The lens frame has an inside diameter a little larger than the diameter of the lenses to be inserted. Finally, a cylindrical clearance between the outer periphery of a lens and the inner periphery of the lens frame is filled with an adhesive.
In the above-described filling of an adhesive to secure the rear lens, since one end of the lens frame has been closed airtightly, it is necessary to fill the clearance with adhesive, to expel the air from the clearance. When adhesive is applied into the clearance, it moves slowly, due to the surface tensions, from the inner periphery of the lens frame and the outer periphery of the lens, thus quickly expelling the air from the clearance. However, when an adhesive is supplied forcedly faster than this air-expelling speed, the air is trapped in the adhesive, inevitably forming many bubbles in the adhesive.
Consequently, when the adhesive hardens and its volume contracts, the air bubbles in the adhesive form pinholes which connect the space, defined by the first objective lens, the rear lens and the lens frame, to the outside, impairing the airtightness of the objective optical system.
Therefore, in filling an adhesive in assemblying objective optical systems, skilled labor is required to fill adhesive in the clearance at a speed lower than the air-expelling speed. In addition, many workhours is required.